Problem: Add. $\begin{aligned} 28& \\ 21&\\ 31&\\ \underline{+46}& \\ \end{aligned}$
First, let's add $28 + 21$. $$ $$ $$ $$ ${+}$ $$ $$ $$ ${=}$ $$ $$ $$ $$ $$ $$ $28} + {21} = 49$ Now, let's add $31$ to $49$. $$ $$ $$ $$ $$ $$ ${+}$ $$ $$ $$ $$ ${=}$ $$ $$ $$ $$ $$ $$ $$ $$ $49+ {31} = 80$ Last, let's add $46$ to $80$. $$ $$ $$ $$ $$ $$ $$ $$ ${+}$ $$ $$ $$ $$ $$ $$ ${=}$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $$ $80 + {46} = 126$ $\begin{aligned} 28& \\ 21&\\ 31&\\ \underline{+46}& \\ 126& \end{aligned}$